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Lindsay B. Baker, Kelly A. Barnes, Bridget C. Sopeña, Ryan P. Nuccio, Adam J. Reimel and Corey T. Ungaro

The collection of athletes’ sweat during training or competition is a common practice in sports science. Sodium ([Na + ]), potassium ([K + ]), and/or chloride ([Cl − ]) concentrations are measured to estimate athletes’ sweat electrolyte losses to determine electrolyte balance or inform personalized

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Hedy C. Reynolds, Loren Cordain, Mary A. Harris and Sheri Linnell

Thirteen trained runners were studied to determine whether postexercise glucose ingestion contributes to electrocardiogram (ECG) alterations by enhancing decreases in serum potassium (K+) concentrations. For the two randomly ordered trials, subjects ingested a 100 g (25% w/v glucose polymer) drink, either alone or with the addition of 3 g of potassium chloride (KCI), within 15 min following a 90-min run. ECG parameters, serum K+, and glucose concentrations were measured preexercise (Time 0), 2-3 min postexercise (Time 1), and 25 (Time 2) and 60 (Time 3) min postexercise. The data suggest that postexercise glucose ingestion may cause ECG changes that are not directly related to the return of K+ to muscle, and that these changes, although characteristic of hypokalemia, may be related to serum glucose excursions rather than to absolute levels of serum K+. The addition of KCl may have prevented these changes by delaying gastric emptying of glucose.

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Fredric Goss, Robert Robertson, Steve Riechman, Robert Zoeller, Ibrahim Dabayebeh, Niall Moyna, Nicholas Boer, Jennifer Peoples and Kenneth Metz

This investigation evaluated the effect of oral potassium phosphate supplementation on ratings of perceived exertion (RPE) and physiological responses during maximal graded exercise tests (GXT). Eight highly trained endurance runners completed a GXT to anchor the Borg 15-point RPE scale and two double-blind counterbalanced GXTs. Subjects ingested either 4,000 mg · day−1 of phosphate (PHOS) or a placebo (PLA) for 2 days. Two weeks separated GXTs. Phosphate levels obtained immediately prior to the GXTs were greater in PHOS than PLA. No differences between PHOS and PLA were noted for the submaximal and maximal physiological responses. RPE for the overall body were lower during PHOS than PLA at intensities corresponding to 70–80% of V̇O2max. This suggests that oral potassium phosphate supplementation mediates perceived exertion during moderately intense exercise.

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Constance B. Christ, Mary H. Slaughter, Rachel J. Stillman, Timothy G. Lohman and Richard A. Boileau

Variability associated with the effects of gender, race, maturation (ML), and age on the potassium content of the fat-free body (K/FFB) was investigated in 163 males and 112 females ages 8 to 30 years. Measures of body density (hydrostatic weighing), bone mineral content (single photon absorptiometry), total body water (modified deuterium dilution), and whole body potassium (40K spectroscopy) were obtained. FFB was calculated using a multicomponent equation which accounted for the variability in FFB mineral and water content. Subjects were classified by maturation as prepubescent, pubescent, postpubescent, and adult (Tanner stages). Least-squares multiple regression analysis revealed significant (p<.05) effects of gender and maturation as well as an interaction between whites and blacks with maturation. The significant increase in K/FFB across maturation and age was most evident in the male sample for both racial groups, with K/FFB increasing from 2.37 in prepubescents to 2.54 in adults. The magnitude of increase in K/FFB across maturation was smaller and not statistically significant within the female sample. Hence, consideration of gender, maturation, and age is important in estimating FFB from total body potassium.

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Mark A. Tarnopolsky, Kerry Dyson, Stephanie A. Atkinson, Duncan MacDougall and Cynthia Cupido

We studied the effects of different CHO supplements on exercise metabolism (1 hr at 75% V˙O2) and performance (fatigue time at 85% V˙O2) in 8 male endurance athletes (VO2max=68.8±3.8 mlkg1min1) Four treatments were administered in a randomized, double-blind fashion: Trial A = 3-day pretest, postexercise supplementation (177 kcal [81% carbohydrate, 19% protein] consumed < 10 min after exercise) + 600 ml 8% glucose polymers/ fructose 1 hr pretesting + 600 ml 8% glucose polymers/glucose during testing; Trial B = placebo during 3-day pretest + remainder same as Trial A; Trial C = placebo at all time points; and Trial D = same as Trial B with 8% glucose 1 hr before the test as well as during the test. Time to fatigue at 85% V˙O2max (Í24%) and total CHO oxidation were greater for A versus C (p < .05). Plasma glucose concentration was higher for A and B versus C, while increases in plasma potassium concentration were attenuated for A versus C (both p < .05). None of the supplements had differential effects upon hematocrit, plasma sodium [Na+] and lactate, V˙O2, or rating of perceived exertion during exercise. Three-day preexercise protein + carbohydrate supplements followed by 1-hr pre- and during-exercise mixed carbohydrate supplements increased time to fatigue and carbohydrate oxidation and attenuated rises in plasma [K+] com pared to placebo.

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Donal Murray, Kevin C. Miller and Jeffrey E. Edwards

Clinical Scenario:

Although exercise-associated muscle cramps (EAMC) are common in ultradistance runners and athletes in general, their etiology remains unclear. EAMC are painful, sudden, involuntary contractions of skeletal muscle occurring during or after exercise and are recognized by visible bulging or knotting of the whole, or part of, a muscle. Many clinicians believe EAMC occur after an imbalance in electrolyte concentrations, specifically serum sodium concentration ([Na+]s) and serum potassium concentration ([K+]s). Studies that have established a link between EAMC occurrence and serum electrolyte concentrations after an athletic event are unhelpful.

Focused Clinical Question:

Are [Na+]s and [K+]s different in athletes who experience EAMC than noncrampers?

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Scott J. Montain, Samuel N. Cheuvront and Henry C. Lukaski

Context:

Uncertainty exists regarding the effect of sustained sweating on sweat mineral-element composition.

Purpose:

To determine the effect of multiple hours of exercise-heat stress on sweat mineral concentrations.

Methods:

Seven heat-acclimated subjects (6 males, 1 female) completed 5 × 60 min of treadmill exercise (1.56 m/s, 2% grade) with 20 min rest between exercise periods in 2 weather conditions (27 °C, 40% relative humidity, 1 m/s and 35 °C, 30%, 1 m/s). Sweat was collected from a sweat-collection pouch attached to the upper back during exercise bouts 1, 3, and 5. Mineral elements were determined by using inductively coupled plasma-emission spectrography.

Results:

At 27 °C, sweat sodium (863 [563] µg/mL; mean [SD]), potassium (222 [48] µg/mL), calcium (16 [7]) µg/mL), magnesium (1265 [566] ng/mL), and copper (80 [56] ng/mL) remained similar to baseline over 7 h of exercise-heat stress, whereas sweat zinc declined 42–45% after the initial hour of exercise-heat stress (Ex1 = 655 [362], Ex3 = 382 [168], Ex5 = 355 [288] µg/mL, P < 0.05). Similar outcomes were observed for sweat zinc at 35 °C when sweat rates were higher. Sweat rate had no effect on sweat trace-element composition.

Conclusions:

Sweat sodium, potassium, and calcium losses during multiple hours of sustained sweating can be predicted from initial sweat composition. Estimates of sweat zinc losses, however, will be overestimated if sweat zinc conservation is not accounted for in sweat zinc-loss estimates.

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Michael F. Bergeron, Carl M. Maresh, Lawrence E. Armstrong, Joseph F. Signorile, John W. Castellani, Robert W. Kenefick, Kent E. LaGasse and Deborah A. Riebe

Twenty (12 male and 8 female) tennis players from two Division I university tennis teams performed three days of round-robin tournament play (i.e., two singles tennis matches followed by one doubles match per day) in a hot environment (32.2 ± 1.5 C° and 53.9 ± 2.4% rh at 1200 hr), so that fluid-electrolyte balance could be evaluated. During singles play, body weight percentage changes were minimal and were similar for males and females (males -1.3 ± 0.8%, females -0.7 ± 0.8%). Estimated daily losses (mmol · day1) of sweat sodium (Na+) and potassium (K+) (males, Na+ 158.7, K+ 31.3; females, Na+ 86.5, K+ 18.9) were met by the players' daily dietary intakes (mmol · day1) of these electrolytes (males, Na+ 279.1 ± 109.4, K+ 173.5 ± 57.7; females, Na+ 178.9 ± 68.9, K+ 116.1 ± 37.5). Daily plasma volume and electrolyte (Na+, K+) levels were generally conserved, although, plasma [Na+] was lower (p < .05) on the morning of Day 4. This study indicated that these athletes generally maintained overall fluid-electrolyte balance, in response to playing multiple tennis matches on 3 successive days in a hot environment, without the occurrence of heat illness.

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Michael F. Bergeron

A 17-year-old, nationally ranked, male tennis player (AH) had been experiencing heat cramps during tennis match play. His medical history and previous physical exams were unremarkable, and his in-office blood chemistry profiles were normal. On-court evaluation and an analysis of a 3-day dietary record revealed that AH's sweat rate was extensive (2.5 L · hr−1) and that his potential daily on-court sweat sodium losses (89.8 mmol · hr of play') could readily exceed his average daily intake of sodium (87.0-174.0 mmol · day−1). The combined effects of excessive and repeated fluid and sodium losses likely predisposed AH to heat cramps during play. AH was ultimately able to eliminate heat cramps during competition and training by increasing his daily dietary intake of sodium.

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Jay L. Tuttle, Jeffrey A. Potteiger, Blanche W. Evans and John C. Ozmun

This study examined the effects of aspartate supplementation (ASP) on plasma ammonia concentrations ([NH4+]) during and after a resistance training workout (RTW). Twelve male weight trainers were randomly administered ASP or vitamin C in a crossover, double blind protocol, each trial separated by 1 wk. ASP and vitamin C were given over a 2-hr period beginning 5 hr prior to the RTW. The RTW consisted of bench, incline, shoulder, and triceps presses, and biceps curls at 70% of one repetition maximum (1-RM). After the RTW a bench press test (BPT) to failure at 65% of 1-RM was used to assess performance. [NH4+] was determined preexercise, 20 and 40 min midworkout, immediately postexercise, and 15 min postexercise. Treatment-by-time ANOVAs, paired t tests, and contrast comparisons were used to identify mean differences. No significant differences were observed between treatments for [NH4+] or BPT. [NH4+] increased significantly from Pre to immediately postexercise for both the ASP and vitamin C trials. Acute ASP supplementation does not reduce [NH4+] during and after a high intensity RTW in weight trained subjects.